In the heat insulation of buildings, by far the weakest points are the windows. Simple double glazings of windows allow more than 3 watts per square meter and degree temperature difference to get lost, while the heat transition coefficient (K-value) of well-insulating walls is under 0.6.
Windows, however, cause not only heat losses; with proper arrangement they also bring an energy gain by the daily irradiation, even in winter and under a cloudy sky. It is generally recognized today that window elements are one of the most important components for passive solar utilization, and it can be foreseen that this finding will find its expression in the future even more in corresponding architectural and structural engineering solutions.
Large-surfaced glazing of residential rooms, however, requires a particularly low K-value, possibly comparable with that of the brickwork; otherwise there is no comfort due to physiological radiation and the increased air circulation in front of the cold glass wall, and the efficiency is offset again by the higher room temperature level.
The insulating glass available today does not nearly exhaust the physical possibilities. While in the last years intensive work has been done to improve the heat insulation of window elements and certain progress has been made, an increase in the solar transmission has not been achieved. On the contrary: the unilateral emphasis on the K-value has led to the development of heat reflecting layers which reduce the total transmission. The data of the manufacturers in this respect are therefore mostly confined to the maximum transmission at a certain wavelength in the visible range. The presently available gold-coated glasses with two panes have a solar transmission of almost 50%. Besides, thin gold coatings produce conspicuous color effects which prevent their wide use in residential buildings for esthetic reasons.
Known three-pane insulating glasses offer in this respect no convincing solution either, since only K-values of about 1.9 W/m.sup.2 K are attained, and the third pane results likewise in marked reduction of the transmission, as well as increased material costs and weight.
The known technique of filling the space between glass panes with a suitable heavy gas, like freon or sulfur hexafluoride, to reduce the thermal conductivity of the unit, can be considered as perfected. Alternate gases which meet all physical and chemical requirements and are also non-toxic, non-flammable, safe and inexpensive are not available.
The situation is similar in selective coating, which is used to reduce the heat radiation of the glass pane and thus to bring about a clear improvement of the K-value. A reduction of the K-value below 1.4 W/m.sup.2 K requires according to today's knowledge the selective coating of at least two glass surfaces of the insulating glass unit, which is not possible on the basis of the known layers for the above-mentioned reasons: Lack of transmission and color neutrality.
Two basic types of selectively transmissive layers are known at present: Transparent semiconductor layers, like doped tin oxide or indium oxide and very thin noble metal films of gold, silver, copper and the like. Despite many production suggestions described in the literature, only gold coating by cathode sputtering could assert itself commercially. Tin oxide layers, as they are produced, for example, by dipping, are not transmissive enough for insulating glass. The use of indium oxide was thoroughly investigated, but had to be given up because of the high processing costs. Other presently known semiconductor materials with usable optical properties are not available.
For the gold coat there is no true alternative either, despite its weaknesses, because this noble metal combines two necessary properties, namely high chemical stability and good electrical conductivity. Other good electron conductors are not so stable (for example copper or silver), likewise not color neutral, or prohibitive in their material costs (for example platinum or rhodium).
It is now being tried to overcome these difficulties by building multi-layer systems of three or more semiconductor and metal films, but it can be foreseen that the possible minor improvements of the optical characteristics will be offset by increasing coating costs, so that no technical progress can be achieved.
It is known that the optical constants of metals or composite materials are influenced by geometric structuring. For example: a reliable and binding measurement of optical constants is so difficult, because the results depend greatly on micro-structural parameters such as particle size, orientation, contamination, surface roughness, oxide film and the like. Despite these generally known relations, no concrete data for the production of selectively transmissive structural filters can be found in the literature.
The following is known from the state of the art: